Obtaining data to enable Ellipsometric analysis of all layers of a sample which comprises a plurality of layers of different materials can be difficult. This is because electromagnetic radiation caused to impinge upon a surface thereof at an oblique angle is generally most sensitive to the outermost layers, particularly if one or more layers is absorbing. For instance, in the case of Three Layers “A”, “B” and “C”, electromagnetic radiation directed at an oblique angle to the surface of the “A” material is not always sensitive to the “C” material, when layers above are substantially opaque so that layers therebelow cannot be investigated therethrough. A solution is to acquire data from the front and back of the sample and use both data sets in analysis, possibly in combination with acquisition of transmission data through the sample using a beam directed substantially along a normal to the top surface, or at an angle with respect thereto. (Note, not all layers need be absorbing for the present invention to be of value).
A Patent to Herzinger et al. U.S. Pat. No. 6,455,853 Issued Sep. 24, 2002, from which this Application is a CIP via application Ser. No. 10/238,241, describes use of data obtained from two sides of a sample. Said 853 Patent taught utilizing Spectroscopic IR and NIR data sets obtained from the front and back of a sample, to evaluate Thickness and Impurity Profile defining parameters therein, as opposed to a sample comprised of a plurality of layers of different materials.
Further disclosed is a U.S. Pat. No. 6,636,309 to Johs et al., from which this Application is a CIP via application Ser. No. 10/194,881. Said Patent describes fabrication of narrow bandpass and band reject stacked filters using beams of electromagnetic radiation which are directed at an oblique angle of incidence to a surface thereof to monitor real time deposition. Said Patent identifies use of an intermediate wavelength band which surrounds, for instance, a narrow passband, in which narrow range reflectivity is nearly constant. It is noted that the materials of the multiple layers in the stacked filter are substantially transparent under the teachings of the 309 Patent, and that the present invention is appropriate for use with multiple layers of materials which are to various degrees, absorbing.
In addition, U.S. Pat. No. 4,472,633 to Motooka is disclosed as it describes use of linearly polarized infrared light to investigate semiconductor wafers. Plots of Ellipsometric PSI vs. Ellipsometric DELTA, as a function of Angle of Incidence and/or Wavelength, for various carrier density profiles and depths are determined. Ellipsometric data obtained from a sample wafer is then utilized to plot Ellipsometric PSI vs. Ellipsometric DELTA, as a function of Angle of Incidence and/or Wavelength, and the results compared to the known plots. Close correlation between sample wafer and a known Ellipsometric PSI vs. Ellipsometric DELTA, as a function of Angle of Incidence and/or Wavelength, is indicative of the sample having a doping profile and depth similar to that of the wafer from which the known Ellipsometric PSI vs. Ellipsometric DELTA data was obtained. Data is described as obtained utilizing monochromatic light, even though different wavelengths are used in succession where wavelength is the independent variable.
Another Patent, U.S. Pat. No. 4,807,994 to Felch et al., describes a non-ellipsometric method of mapping ion implant dose uniformity. Monochromatic Electromagnetic radiation with a bandwidth of not more than 1 nm, (chosen for sensitivity to sample parameters being measured), which has interacted with a sample in Reflectance or Transmission, is monitored by a Spectrophotometer and the results compared to previously obtained similar data regarding film thickness and ion implant doses, and similarities determined.
U.S. Pat. No. 5,486,701 to Norton et al., describes a non-ellipsometric approach simultaneously utilizing wavelengths in both UV and Visible wavelength ranges to enable calculating a ratio thereof, which in turn is utilized to determine thin film thicknesses.
U.S. Pat. No. 6,049,220 to Borden et al., describes apparatus and method for evaluating semiconductor material. In a major implementation thereof, two beams are caused to illuminate a sample, one having energy above the bandgap and the other having energy near or below the bandgap. The second beam, after interaction with the sample, is monitored and change therein caused by said interaction is indicative of carrier concentration. It is noted that reflectance of an electromagnetic beam from a sample is a function of carrier concentration.
U.S. Pat. No. 5,625,455 to Nash et al. and U.S. Pat. No. 5,900,633 to Solomon et al. were provided by the Examiner in Examination of Parent application Ser. No. 09/756,515, (now U.S. Pat. No. 6,455,853).
A Patent to Herzinger et al., U.S. Pat. No. 5,796,983 is identified, and incorporated by reference herein, as it describes Parameter Models which can be applied in practice of the present invention.
Known relevant art includes Articles:
“P-N Junction-Based Wafer Flow Process For Stencil Mask Fabrication”, Rangelow et al., J. Vac. Sci. Technology B, Nov/Dec P. 3592 (1998); and
“Application of IR Variable Angle Spectroscopic Ellipsometry To The Determination Of Free Carrier Concentration Depth Profiles”, Tiwald et al., Thin Film Solids 313-314, P661, (1998).
“Optical Analysis of Complex Multilayer Structures Using Multiple Data Types”, Johs et al., SPIE Vol. 2253, (1994). This paper describes use of ellipsometric data obtained from both sides of a fused silica substrate which has a layer of chromium on one surface thereof, in combination with transmission intensity data, to determine optical constants and thicknesses. This paper, however, does not address the case in which a plurality of layers of material are present on at least one surface of a sample, and it is desired to determine optical constants and thicknesses for each, and a data set consisting of two PSI and two DELTA values, (obtained by ellipsometric investigation from both sides of a sample), plus a transmission data set, is insufficient to directly evaluate optical constants and thicknesses for each layer present at each wavelength. The 853 Patent identified above, and from which this Application is a CIP via Pending application Ser. No. 10/194,881, provides the necessary seed of additional insight which has led to the present invention.
In addition, a J. A. Woollam Co. Inc. 2002 flyer is identified, as it included an article on global fitting to provide starting values for entry to a regression procedure.
A need remains for a method of utilizing data sets to practically determine optical constants and thicknesses for each layer in a multiple layer sample, which data set, on a wavelength by wavelength basis, is insufficient to provide accurate determination of all optical constants and thicknesses for each of a plurality of layers on a substrate.